This invention relates to the anaerobic treatment of feed stocks to generate biogas, and more particularly to the anaerobic treatment of nitrogen containing feed stocks to generate a biogas.
Over the past several decades, extensive scientific and engineering work has been conducted on the biogasification of waste materials. The fundamental technique relies on the anaerobic digestion or fermentation process. Anaerobic digestion of biomass materials proceeds in three distinct and sequential pathways. These pathways are hydrolysis, acetogenesis and methanogenesis. The anaerobic microorganisms that conduct the first two steps, the hydrolyzers and acetogens, break the complex biomass molecules down into small chain molecules. Proteins are hydrolyzed into proteoses, peptones and polypeptides. These compounds are further broken down into ammonia and small chain fatty acids such as acetic acid, butyric acid, propionic acid, and lactic acid. The anaerobic microorganisms that perform the hydrolysis and acetogenesis functions are highly resistant to ammonia. Anaerobic fermentation of high nitrogen wastes using these microorganisms have produced digested streams containing in excess of 10,000 ppm ammonia. However, the anaerobic microorganisms responsible for methanogenesis are inhibited by ammonia. Methanogenesic anaerobic bacteria cease to function effectively at ammonia concentrations equal to or greater than approximately 1,200 ppm ammonia. (Kayhanian, M., Environmental Technology, Vol. 20, PP 355-365, 1999)
Technologies such as the Upflow Anaerobic Sludge Blanket (UASB) reactor and the Extended Granular Sludge Bed (EGSB) reactor offer advantages in the anaerobic fermentation or digestion of wastewater or other feed stocks. These reactors allow for higher treatment rates using smaller vessels, thereby reducing capital costs. These reactors also provide for improved odor control. Still, problems associated with ammonia inhibition have made these reactors relatively unstable and difficult to operate when using feed stocks containing relatively high concentrations of nitrogen. To mitigate these problems, it has been proposed to control the carbon to nitrogen (C/N) ratio of the feed stock and to dilute the reactors with water in cases of sudden, large ammonia overloads. These proposals still suffer from a number of disadvantages. For example, adjusting the ammonia concentration in a reactor by adjusting the C/N ratio of the feed stock is a slow process, it can be difficult to accurately determine the C/N ratio, and adjusting the C/N ratio may prove to be insufficient to handle feed stocks that are prone to generate relatively high ammonia concentrations during anaerobic digestion. Dilution of a reactor with water also has a number of disadvantages. For example, diluting the reactor with water may seriously decrease the reactor""s biogas production for extended periods of time and will typically lead to increased dewatering costs. Dilution of an existing feed stock increases the required reactor volume for digestion of that feed stock. An existing reactor would have a decreased capacity for treating a given feed stock if that feed stock were diluted.
It is therefore an object of the present invention to provide a stable system for treating nitrogen containing biomass materials to generate a biogas.
It is a further object of the present invention to provide a system of the above type that provides for separation of the anaerobic digestion process so that methanogenesis takes place in a separate reactor or other receptacle.
It is a still farther object of the present invention to provide a system of the above type in which ammonia removal prior to methanogenesis keeps ammonia levels sufficiently low to avoid ammonia inhibition problems.
It is a still further object of the present invention to provide a system of the above type in which the system is operated under mesophilic conditions.
It is a still further object of the present invention to provide a system of the above type in which the system is operated under thermophilic conditions.
It is a still further object of the present invention to provide an alternate embodiment of a system of the above type in which effluent from a reactor is treated for ammonia removal and recycled to the reactor at a rate sufficient to keep ammonia levels within the reactor sufficiently low to avoid ammonia inhibition problems.
Toward the fulfillment of these and other objects and advantages, the system of the present invention comprises a first anaerobic digester, an ammonia recovery vessel, and a second anaerobic digester. Microorganisms within the first anaerobic digester are primarily hydrolyzers and acetogens, and microorganisms within the second anaerobic digester are primarily methanogens. A nitrogen containing feed stock is passed to the first digester in which the feed stock is treated to accomplish hydrolysis and acetogenesis. An effluent stream from the first digester is passed to the ammonia recovery vessel in which ammonia is removed to generate a low ammonia effluent stream. The low ammonia effluent stream is then passed to the second digester in which it is treated to accomplish methanogenesis, thereby generating a biogas. In an alternate embodiment, a single anaerobic digester is used, an effluent stream is removed from the reactor, treated for ammonia removal, and recycled to the digester at a rate sufficient to keep ammonia levels within the digester sufficiently low to avoid ammonia inhibition problems. The systems may be operated under mesophilic or thermophilic conditions.